Switch to using HashChangeEvent constructor instead of initHashChangeEvent

sdk/lib/html/dartium/html_dartium.dart

Index: sdk/lib/html/dartium/html_dartium.dart
diff --git a/sdk/lib/html/dartium/html_dartium.dart b/sdk/lib/html/dartium/html_dartium.dart
index 0d1edad2b9d7903acc957bfd2ddb45ba83e0d33b..6199c17c887519cab358032b05222129bd26e54a 100644
--- a/sdk/lib/html/dartium/html_dartium.dart
+++ b/sdk/lib/html/dartium/html_dartium.dart
@@ -19702,10 +19702,14 @@ class HashChangeEvent extends Event {
   factory HashChangeEvent(String type,
       {bool canBubble: true, bool cancelable: true, String oldUrl,
       String newUrl}) {
+
+    // TODO(alanknight): This is required while we're on Dartium 39, but will need
+    // to look like dart2js with later versions when initHashChange is removed.
     var event = document._createEvent("HashChangeEvent");
     event._initHashChangeEvent(type, canBubble, cancelable, oldUrl, newUrl);
     return event;
   }
+
   // To suppress missing implicit constructor warnings.
   factory HashChangeEvent._() { throw new UnsupportedError("Not supported"); }